Lymphatic System

• Lymphoid Tissues

  • Lymphoid Tissue
• Lymphoid Tissue

  • Aggregates of lymphocytes in different regions of the body
  • Parenchyma: Lymphocytes
  • Stroma: Reticular tissue
  • Other cells: Plasma cells, macrophages, neutrophils, eosinophils
• Reticular Fibers and Cells

  1. Functions: Protect internal environment from invasion and damage by foreign substances, cells or microorganisms

  2. Components of the immune system
• Lymphocytes

  • Small, spherical or oval
  • Large rounded nucleus or slightly indented on one side
  • Narrow non-granular cytoplasm
• Lymphocytes

  • Small lymphocytes: Majority of cells, 6-9 um, Dark staining
  • Large lymphocytes: 9-15 um, Pale
  • Lymphoblasts
• Lymphocytes

  1. Antigenic stimulation sensitizes small lymphocytes

  2. Small lymphocytes recognize foreign proteins and store this information

  3. Both the T and B-lymphocytes are referred to as memory cells

  4. Programmed to recognize and react with specific types of antigen
• Classification of Lymphoid Tissue

  • Loose lymphoid tissue
  • Dense lymphoid tissue
  • Nodular lymphoid tissue
• Loose Lymphatic Tissue

  • Few lymphocytes
  • Irregularly and loosely scattered
  • Stroma: reticular cells and reticular fibers
  • Examples: Trachea, Esophagus, Internodular, deep cortical and medullary sheaths of the spleen, Internodular regions of the tonsils and Peyer patches
• Dense Lymphatic Tissue

  • Lymphocytes are abundant and closely packed
  • Examples: Ileum, Colon, Vermiform appendix
• Nodular Lymphoid Tissue

  • Compact, circumscribed aggregations of lymphocytes
  • Aka lymphoid follicles
  • Not permanent structures
  • Solitary lymphoid nodules: Lamina propria of the digestive, respiratory, and urinary tract, Aggregates of lymphoid nodules, Walls of the ileum called Peyer patches, Vermiform appendix
• Germinal Center


  • A central, pale staining portion of a lymphoid nodule, Composed of bigger lymphocytes often with mitotic figures in normal subjects, Germinal center of Fleming, Secondary nodule, Corona, Peripheral, dark staining portion of lymphoid nodules, Small lymphocytes with dark staining nucleus and closely packed cells
• Germinal Center

  Site of active production of lymphocytes, Trap antigen in the presence of antibodies to retain antigen-antibody complex, Involved in functional differentiation of B-lymphocytes, Related to long term memory response of IgG antibody
• LYMPHOID ORGANS

  • Thymus
• Thymus

  • Capsule: Loose connective tissue, Trabeculae (septae) subdivides the thymic lobes into lobules, Parenchyma: Lymphocytes (thymocytes), Stroma: Epithelial reticular cells, Macrophages
• Thymus

  • Thymic Cortex: A peripheral zone of dense lymphoid tissue consisting of T-lymphocytes, Do not form lymphoid nodules, Blood supply: Capillaries, Active site of lymphocyte production, Epithelial reticular cells are less numerous, Stellate in shape, Scanty acidophilic cytoplasm, Large, oval nucleus
• Thymus

  • Thymic Medulla: Few pale staining lymphocytes, Reticular cells, Extremely pleomorphic and have more acidophilic cytoplasm, Hassall’s bodies: Spherical or oval bodies, Concentrically arranged cells of epithelial reticular cells with an acidophilic central area
• Thymic Medulla

  Development of the class of lymphocytes responsible for cellular or cell-mediated immunological response
• Thymic Cortex

  • Extremely pleomorphic and have more acidophilic cytoplasm
  • Contain Hassall’s bodies
  • Contain spherical or oval bodies
  • Contain concentrically arranged cells of epithelial reticular cells with an acidophilic central area
• Thymic Medulla

  1. Development of the class of lymphocytes responsible for cellular or cell-mediated immunological response

  2. Homograph rejection

  3. Delayed cutaneous reactions to protein antigens (delayed hypersensitivity)

  4. Graft versus host reaction

  5. Immune response to fungi, bacteria, and certain viruses

  6. Supplies lymphocytes to the secondary lymphoid organs
• Medulla of the Thymus

  • Functions related to lymphocyte development and immune responses
• Lymph Nodes

  • Encapsulated, spheroid or kidney-shaped organs along the course of lymphatic vessels
  • Contain hilus and indentation on one side where blood vessels enter or leave
  • Consist of afferent and efferent lymphatic vessels with specific valve arrangements for unidirectional lymph flow
  • Composed of dense connective tissue capsule, stroma, reticular tissue, lymphocytes, plasma cells, macrophages, and lymph sinuses
• Lymph Node Cortex

  • Outer, densely staining portion containing lymphoid nodules with germinal centers, deep or inner cortex composed of diffuse lymphoid tissue mainly T-lymphocytes
• Lymph Node Medulla

  • Contains medullary cords, reticular fibers, reticular cells, small lymphocytes, mature plasma cells, and macrophages
• Lymphatic Pathway and Sinuses

  Describes the pathway of lymph through a lymph node including afferent and efferent lymphatic vessels, subcapsular sinus, cortical sinus, medullary sinus, and the prevention of backflow by valves
• Functions of Lymph Nodes
• Functions of Spleen
• Spleen White Pulp

  • Forms periarteriolar lymphoid sheath (PALS) around arteries, may contain germinal centers, and consist of T-lymphocytes
• Components of the internal supporting framework

  • Trabeculae
  • Fibrous extensions into the organ
  • Reticular tissue
• White Pulp of the Spleen

  • Forms periarteriolar lymphoid sheath (PALS)
  • Around arteries leave the trabeculae to penetrate the parenchyma
  • May contain germinal centers
  • Consists of T-lymphocytes
  • Germinal centers consist of B-lymphocytes
• Red Pulp of the Spleen

  • Surrounds the white pulp
  • Consists of plexuses of venous sinuses
  • Subdivided into splenic cords of Billroth
  • Contains a large number of RBC’s
  • Responsible for its color in fresh preparations
  • Contains macrophages, erythrocytes, platelets, and a few plasma cells
• Marginal zone


  • Transitional region between the white pulp and the red pulp
  • Contains smaller venous sinuses
  • Receives the incoming arterial blood
  • Site where blood-borne cells and particulate matter first contact the splenic parenchyma
  • Lymphocytes of the recirculating pool leave the blood of the sinuses to enter the PALS
• Arteries in the Spleen

  • Branches of the splenic artery enter the hilus and pass along the trabeculae
  • Blood flow in the spleen
• Venous Sinuses and Veins in the Spleen

  • Venous sinuses permeate the entire red pulp and are numerous around the white pulp
  • Venous sinuses empty into the veins of the pulp
  • Pulp veins coalesce to form the veins of the hilus of the spleen
• Functions of the Spleen

  • Graveyard for worn out RBC’s
  • Reservoir of RBC’s
  • Formation of lymphocytes
  • Removal of particulate matters in circulation
  • Immune response to bacteria, viruses, and foreign macromolecules
• Erythrocyte Removal by Splenic Sinuses


  Process of removing worn out RBC’s
• Palatine Tonsils

  • Paired oval-shaped bodies located in the oropharynx
  • Consist of dense accumulations of lymphatic tissues
  • Covered by stratified squamous non-cornified epithelium
  • Contain tonsillar crypts
  • Surrounded by dense lymphoid tissue with lymphoid nodules
• Capsule of Palatine Tonsils

  Dense connective tissue over the attached or basal surface of the tonsils
• Salivary Corpuscles

  • Lymphocytes and neutrophils found in saliva
  • Appear as degenerating vesicular elements with a pyknotic nucleus surrounded by a clear vesicle containing granules